Physical layer multi-antenna technique has become one of key techniques in the next generation wireless communication systems. The multi-antenna technique has many advantages, such as increasing system throughput by multiplexing gain of multiple antennas, improving system performance by diversity gain of multiple antennas, eliminating multiuser interference between different receiving terminals by directional beamforming gain of the antennas, and so on.
MIMO (Multiple-Input Multiple-Output) systems use multiple transmit and receive antennas to obtain capacity, diversity and array gain by jointly processing signals in space, time and frequency domains. In a MIMO system, if a transmitter can obtain channel information in a certain way, transmission signals can be optimized by using channel characteristics to improve receiving quality and reduce the requirements for receiver complexity. In practice, quantized channel information is adopted for feeding back channel information to reduce feedback overhead and feedback delay. The quantized channel information may be a channel matrix or statistics of the channel matrix, or a parameter for pre-processing recommended by the receiver. Quantized channel information is mapped onto a set composed of a limited amount of elements which is referred to as a codebook.
In a codebook-based closed loop MIMO system, a receiving end receives signals transmitted by a transmitting end, and converts the signals into frequency domain signals by FFT (Fast Fourier Transform). Then the signals are divided into several layers (or streams) by a MIMO detection module. After de-layer mapping, the signals are then demodulated, de-interleaved and decoded. The received pilot signals are inputted into a channel estimation module for channel estimation. The receiving end selects a precoder from the codebook based on channel estimation information to optimize the performance. The optimization criterion may be FER (Frame Error Rate), BER (Bit Error Rate), SINR (Signal to Interference plus Noise Ratio), mutual information, instantaneous throughput, or other proper criteria. The receiving end generates an index of the selected precoder for subsequent transmission of the transmitting end. The receiving end generates a CQI (Channel Quality Indication) to inform the transmitting end of the quality of the channel. The receiving end may further determine a rank and the number of data layers corresponding to the rank for subsequent data transmission. The rank is the number of layers of a signal transmitted in the MIMO system. All of the above obtained parameters, including the precoder index, the CQI and the rank, are fed back to the transmitting end. The transmitting end processes multiple to-be-sent data streams by using a selected MCS (Modulation and Coding Scheme). A precoder is selected from the codebook according to precoder selection information. Each data stream is mapped to several layers to achieve a required transmission data rate. Then, the data after layer-mapping is pre-coded using the selected precoder. Finally, each data stream after precoding is modulated by using OFDM (Orthogonal Frequency Division Multiplexing), and then transmitted via multiple transmit antennas.